Idiopathic thrombocytopenic purpura appears to result from an immunologic reaction against a platelet associated antigen(s) with subsequent production of an antiplatelet antibody. The present proposal describes a comprehensive approach for evaluating patients with immune thrombocytopenia. The presence of a platelet or megakaryocyte "autoantigen" will be evaluated by determining leukocyte blastogenesis and leukocyte migration from capillary tubes in the presence of autologous platelets or megakaryocytes. Ig production by leukocytes from four lymphoid areas in ITP patients (spleen, bone marrow, lymph nodes, and blood) will be quantitated using the Fab-anti Fab Ig assay system and compared to data from control subjects. Antiplatelet reactivity of autologous sera, lymphoid organ eluates and Ig produced in culture will be evaluated in vitro using three assay systems: platelet Ig binding, platelet factor 3 release, and platelet phagocytosis, as well as, in vivo using autologous platelet survival after in vitro incubation with plasma or culture Ig. These experimental data will be correlated with the patients' clinical course and therapeutic status. A similar experimental approach will be employed in studying the secondary immune purpuras.